The Seattle Water Department (SWD) service area and watersheds lie in seismic zone 3, in which there is potential for major earthquakes (Richter magnitude 7.5 or greater). The SWD system was initiated at the turn of the century, with the result that many of the facilities are more than 50 years old and were designed prior to the existence of seismic design requirements in the building codes. Because of the importance of maintaining an operable water system after a major earthquake, in late 1988 the SWD undertook a comprehensive assessment to determine the seismic reliability of their water system. This assessment is one of the largest of its kind to have been undertaken for a major metropolitan water supply system. This paper describes the methodology by which the vulnerability assessments were conducted. It discusses the level of earthquake ground motion used to conduct the facility assessments along with key findings and observations. Based upon the conclusions and recommendations resulting from this study, the SWD is now undertaking a 6-year program to retrofit and upgrade all of its major facilities, along with its emergency response plan and communications systems.